GaN has a direct bandgap of 3.4 eV at room temperature and, if combined with indium nitride (InN) or aluminum nitride (AlN), can have a direct bandgap of 1.9 eV (InN) or 6.2 eV (AlN). Due to its wide wavelength range spanning the visible light to ultraviolet regions of the electromagnetic spectrum, GaN is a desirable material for light-emitting element applications. Since the material's emission wavelength can be controlled over a wide range, GaN can be used to make light-emitting elements capable of emitting over a wide spectrum of visible light. Consequently, the market for GaN is huge, and includes full color electric signs using red, green and blue light-emitting elements, and other illuminators using a white light-emitting element. As a result, extensive research on the GaN semiconductor has been conducted. In particular, GaN has attracted significant attention as a light-emitting element of a blue light-emitting diode (LED), a blue laser diode (LD), and similar devices in the short wavelength region.
However, there hardly exists a substrate that is lattice matched with this GaN semiconductor, and differences in lattice parameters and thermal expansion coefficients therebetween are too large. Therefore, it is very difficult to grow a high-quality nitride semiconductor thin film.
In general, a sapphire (Al2O3) or silicon carbide (SiC) substrate is used for growing a GaN semiconductor thin film. The theoretical lattice parameter of sapphire in the a-axis is 4.758 Å, while the lattice parameter of GaN is 3.186 Å. That is, their theoretical lattice parameters are different from each other by more than 30%. Consequently, a tensile strain is caused when GaN grows on a sapphire substrate. However, when GaN grows on actual (0001) sapphire, an effective lattice parameter of sapphire is smaller than that of GaN by about 14%, which causes a compressive strain. Further, since their thermal expansion coefficients are different from each other by about 25%, the stress is generated at the interface between the sapphire and GaN. Moreover, since lattice defects having large threading dislocation density of about 1014/cm2 are caused, it is difficult to achieve the growth of a high-quality single crystal. In addition, since a crack can be caused when the thickness exceeds 10 μm, it is difficult to grow a high-quality GaN thin film layer, and the life and characteristic efficiency of devices made therefrom can also be reduced.
However, by successful growing a GaN thin film having excellent electric and optical characteristics and crystallinity on a sapphire substrate after forming an AlN buffer layer on the sapphire substrate, the above problems have been lessened to some extent. Then, various materials such as GaN or AlGaN, in addition to AlN, can be used for forming the buffer layer, and large differences in lattice parameters and thermal expansion coefficients can also be relieved by means of the heteroepitaxy in which a GaN thin film grows using the buffer layer.
Nevertheless, when the buffer layer for the heteroepitaxy is grown to about 1 to 500 nm, and the GaN thin film grows on the grown buffer layer, the buffer layer includes crystal defects due to differences in thermal expansion coefficients and lattice parameters of the substrate and buffer layer. The crystal defects are directly transferred to the GaN thin film. As a result, the large number of crystal defects make it difficult to form a high-quality GaN thin film.
In addition, although a sapphire substrate can be manufactured to have a 4-inch diameter, 2-inch substrates are widely used. This manufacturing preference has a negative impact on the productivity of manufacturing LEDs or LDs in large quantities. A need exists for high-quality substrate that can be manufactured with a large diameter, as a viable replacement for sapphire substrates.
Moreover, sapphire is an insulating material because it is an oxidized material. Therefore, for a device manufactured using 2 sapphire substrate, the back ohmic contact cannot be easily formed, as compared with other devices manufactured using conductive substrates. Accordingly, an additional process or post-process is required for forming electrodes while manufacturing the device, which increases the complexity of the overall manufacturing process. As a result, manufacturing costs are increased, and the performance of devices can be degraded due to the increased serial resistance of the device.
More recently, a high-quality, 2-inch substrate made of SiC has become commercially available. The lattice parameter of SiC is different from that of GaN by 3.3%, which is smaller than the difference of 13.8% between the sapphire and GaN. Thus, SiC is preferable to sapphire in view of lattice matching. In addition, since SiC has excellent chemical stability and high-temperature characteristics, it attracts considerable attention. However, SiC is more expensive than sapphire, and the performance of the GaN device using a SiC substrate is reported to be inferior to that using a sapphire substrate.
Although substantial research has been conducted on the growth of the GaN thin film using a silicon substrate as an alternative to sapphire or SiC substrates there remain problems due to the differences in the lattice parameters and thermal expansion coefficients occurring in the conventional sapphire and SiC substrates. The silicon substrate has an advantage in that the manufacturing costs can be reduced by securing high-quality substrates with a large diameter at a low price, and the applicability to a variety of devices and circuits can also be improved through the integration in addition to the manufacture of a single optical device. However, the silicon substrate still has problems to be solved if more stable devices having reproducible characteristics are to be produced. The difference in the lattice parameter between the GaN thin film and silicon is about 18%, and the defect density of 1010/cm2 is reported to exist in the GaN thin film formed on the silicon substrate. Above all, it is important to allow the high-quality GaN thin film to grow by decreasing the lattice defects due to the difference in the lattice parameter.